α-Olefins (AOs) are important synthetic organic chemicals, being used widely as chemical intermediates and monomers for polymerizations. They are most commonly made by oligomerization of ethylene using transition metal containing or other types of catalysts, see for instance B. Elvers, et al., Ed. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A13, VCH Verlagsgesellschaft mbH, Weinheim, 1989, p. 243-247 and 275-276, and B. Cornils, et al., Applied Homogeneous Catalysis with Organometallic Compounds, A Comprehensive Handbook, Vol. 1, VCH Verlagsgesellschaft mbH, Weinheim, 1996, p. 246-256. Catalysts that may be used include iron complexes of certain diimines of a 2,6-diacylpyridine or a 2,6-pyridinedicarboxaldehyde, see for instance U.S. Pat. No. 6,103,946. The apparent key factor in these complexes making AOs from ethylene instead of higher polymers is control of the steric hindrance about the iron atom, for instance by controlling the size of the aryl group which is of the imino groups, see George J. P. Britovsek, et al., Chem. Eur. J. vol. 6, p. 2221-2231 (2000), which is hereby included by reference. Typically these catalysts are very active, and under the proper conditions these catalysts produce a series of AOs of the general formula CH2═CH2(CH2CH2)nH wherein n is an integer of 1 or more. The relative concentrations of the various AOs produced are determined by the Schulz-Flory equation (SFE), for more details see below.
However it has been found that certain of these iron complexes, for instance those which are symmetrical in substitution of the aryl imino group, produce a mixture of AOs that deviates from the product distribution predicted by the SFE, producing higher amounts of relatively low molecular weight (MW) and higher MW AOs, see U.S. Pat. No. 6,710,006, which is hereby included by reference. While production of small amount of “excess” low MW AOs is not a serious problem, production of “excess” higher MW AOs can be, since they may precipitate from the process mixture and foul the reactor train.
As a solution to this problem the inventors of U.S. Pat. No. 6,710,006 found that similar unsymmetrically substituted ligands gave much better agreement with the SFE and decreased the amount of higher MW AOs produced. Formula (I) is an example of a symmetrical ligand from U.S. Pat. No. 6,710,006, while Formula (II) is an example of their improved ligand.
However, production of the unsymmetrical iron complexes such as (II) requires more steps and more careful purification of the resulting intermediate and final products, so overall yields of the iron complexes are relatively low, 6-17% reported in the three instances in U.S. Pat. No. 6,710,006. Since 2,6-diacylpyridines and 2,6-pyridinedicarboxaldehydes are expensive (for instance 2,6-diacetylpyridine from Aldrich-Sigma is $345.50 for 25 g as of Jul. 30, 2009), such catalysts are relatively expensive.
It would therefore be advantageous if more readily synthesized ligands (and their iron complexes) could be used without giving product mixtures which deviate from the SFE.